1. Field of the Invention
The present invention relates to a ball screw, and more particularly to a ball screw with sectional circulating assemblies, which can be easily assembled and produces less noise.
2. Description of the Prior Art
Nowadays, ball screw is used more and more widely in modern industries. As a linear transmission means, the ball screw not only has high precision but also has the advantages of low frictional loss, high energy conversion ratio, low noise and high rigidity. Hence, it is self-evident that the ball screw is very important to various industrial mechanisms. In order for the rolling balls between the screw shaft and the nut to circulate smoothly and for relieving the stress therebetween, the nut is usually provided at both ends thereof with two circulation cassettes for cooperating with the threads of the screw shaft and the rolling balls. And the circulation cassette is a key structure for enabling the rolling balls circulation from the screw shaft and the nut. Therefore, how to develop an improved and competitive circulation cassette is the common goal that the manufactures are striving for.
The conventional circulating assemblies are essentially classified by way of assembly into two categories: axial assembly (as shown in FIG. 1), and radial assembly (not shown). Please refer to the following detailed description:
Axial Assembly:
FIG. 1 shows a ball screw of axial assembly type disclosed in U.S. Pat. No. 4,357,838, which comprises: a nut 10, a screw shaft 11, a plurality of rolling balls 12 and a circulating element 13. The circulating element 13 is installed into the end surface of the nut 10 for guiding the rolling balls 12 into the circulation hole (not shown) of the nut 10. A very important portion of the circulating element 13 is called nail portion 14 that straddles the threads of the nut 10 for smoothly jointing the helical grooves. This axial assembly has the following disadvantages:
First, although this circulating element 13 has the advantages of smooth rolling path and high rotation speed, since the nail portion 14 is engaged in the helical groove 101 of the screw shaft 11, any end of the helical groove 101 must be open so as to facilitate the installation of the nail portion 14 of the circulating element 13. In other words, the helical groove must extends to the shoulder portion of the screw shaft 11, and the helical groove leaves a notch in the in the abutting surface of the shoulder portion, as a result, the integrality of the diameter of the abutting surface is damaged (the outer periphery of the abutting surface is not round). When both ends of the screw shaft 11 are mounted on bearing seats of a machine (not shown), and the motor of the machine starts to rotate the screw shaft 11, the rotation inertia will effect an axial load on both ends of the screw shaft 11. And due to the integrality of the diameter of the abutting surface is damaged, the axial load cannot be evenly distributed on the both ends of the screw shaft 11. This will generate great pressure against the end surface (not shown) of the bearing seats, causing damage to the bearing seats, as a result of that, the screw shaft 11 will be deformed during rotation, adversely affecting the circulation of the rolling balls, but will reduce the service life of the screw shaft. Therefore, it is uneconomical.
Second, the screw shaft 11 is initially inserted at a proper position in the nut 10, and the rolling balls 12 are placed therebetween, and finally the circulating element 13 should be installed at an appropriate angle, otherwise it can be assembled. Therefore, the assembly of the circulating element 13 is complicated and requires a skilled worker.
Third, when both ends of the screw shaft 11 are mounted on bearing seats of a machine, the screw shaft 11 is then inserted through the nut 10 and filling rolling balls, the ball screw is assembled by hand. The ball screw probably needs to be subjected to adjustment process (the respective components of the ball screw should be adjusted) during assembly, because of the adjustment process, the rolling balls may drop out of the ball screw when the nut 10 is disengaged from the screw shaft 11 or when the nut 10 moves out of travel.
Radial Assembly:
To overcome the abovementioned problems, another radial assembly was developed, wherein a radial hole is defined in the nut for passage and installation of the circulating element during assembly. Although it can overcome the interference between the nail portion of the conventional circulating element and the helical groove of the screw shaft, this radial assembly still has other problems:
First, the radial hole of the nut breaks the tightness of the nut, sot that the vibration noise generated during the rolling motion of the rolling balls will rush out without reserve.
Second, the radial hole assembled circulating element is so stable as the axially assembled circulating element and is more likely to loosen, and even worse, it will damage the ball screw structure.
FIGS. 2 and 3 show a ball screw of axial assembly type disclosed in US Pub. No. 2004/0211280 (Nishimura) discloses a ball screw with a circulating assembly 50, wherein the circulating assembly 50 includes an inner circulating block 9 and an outer circulating block 54. The inner circulating block 9 is provided with a nail portion 541 and a semicircular connecting portion 542 protruding out of the inner circulating block 9. The outer circulating block 54 is also formed with a semicircular connecting portion 543 which cooperates with the semicircular connecting portion 542 to form a protruding circular connecting portion to be inserted in the circulating hole of the nut. The problem of Nishimura is that when the inner circulating block 9 is engaged in a recess 47 of the nut 4, the semicircular connecting portion 542 will simultaneously be inserted in the circulating hole of the nut 4, and the semicircular connecting portion 542 is not movable in a radial direction but only movable in an axial direction with respect to the circulating hole of the nut 4, which will stop the inner circulating block 9 from a radial movement with respect to the nut 4, namely, the inner circulating block 9 is not adjustable in a radial direction with respect to the nut 4 when assembled in the recess 47 of the nut 4. Hence, in assembly, the nail portion 541 of the inner circulating block 9 will interfere with the screw shaft, and as a result, the structure of the circulating assembly 50 of Nishimura is only applicable to the ball screw whose helical groove extends to the shoulder portion thereof.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.